Research Report 2020
3.4
Institute of Computational Physics
Investigation of the Efficiency and the Lifetime in OLEDs
In the research project TripLED, promising OLEDs fabricated by Professor Jang-Joo Kim's group from Seoul National University were investigated. Using different, partly newly developed, characterization methods in combination with electro-optical 1D simulations, improvements in efficiency and lifetime could be predicted. M. Regnat, K. Pernstich, B. Ruhstaller Prof. Jang-Joo Kim Gruppe von der Seoul National University, Sßdkorea SNF 2016–2019
Organic light-emitting diodes (OLEDs) can be nowadays found in displays of mobile phones, televisions and lamps for illumination. Current research is being conducted on the next generation of OLEDs using thermally activated delayed fluorescence (TADF) in order to achieve increases in efficiency with sufficient lifetime. In a first step of this project, such TADF OLEDs have been investigated using different characterization methods with the Paios measurement system [1] and electro-optical 1D simulations with Setfos [2]. As a key result, different routes to enhance the efficiency in these OLEDs by up to 400 % (at 1000 cd/m2) could be demonstrated, as shown schematically in figure 1 [3].
a quantitative electro-optical model was developed to describe the degradation over the lifetime. The decrease of the current-voltage characteristics in the investigated period was mainly due to the increase of traps in the hole transport layers of these TADF OLEDs (see figure 2).
30 20
EQE (%)
- 62% After 1 Months After 2 Months After 4 Months
0
Intersystem crossing rate
0
25
2
4
6
8
Voltage [V]
No host TTA
Figure 2: Measured and simulated current-voltage curves of the fresh and degraded TADF OLEDs. The inset shows the increase of the trap densities in the hole transporting layers during the investigated period.
PL quantum yield Emitting dipole orientation S/T energy difference
15
Started
Simulations Measurements
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30
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Trap Density HTL 2 [1/m3]
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+340%
After 4 Months
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+295%
After 1 Months After 2 Months
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Started
Current Density [mA/cm2]
70 Trap Density HTL 1 [1/m3]
Contributors: Partner(s): Funding: Duration:
10 5 0 100
In this project we have demonstrated that the combination of different characterization methods with electro-optical 1D simulations can be a valuable tool for increasing the efficiency and the lifetime of OLEDs.
Fabricated TADF OLED
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102
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104
Luminance (cd/m2) Figure 1: Possible routes to enhance the external quantum efficiency (EQE) of the investigated TADF OLEDs.
References: [1] Setfos 5.0, https://www.fluxim.com/setfos-intro/. [2] Paios, CS 4.2, https://www.fluxim.com/paios. [3] Regnat, Markus, et al, Advanced Electronic Materials (2019): https://doi.org/10.1002/aelm.201900804
In a further step, the lifetime of highly efficient TADF OLEDs was investigated. For this purpose, they were operated under constant current and characterized at regular intervals. Based on these measurements,
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24
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